1. Field of the Invention
The present invention relates to floating offshore platforms in general, and in particular, to an adjustably buoyant, deep draft, semi-submersible platform for off-shore oil and gas drilling and production operations.
2. Description of Related Art
Conventional shallow draft semi-submersible offshore platforms are used primarily in offshore locations where water depth exceeds about 300 feet (91 meters). This type of semi-submersible platform comprises a hull structure that has sufficient buoyancy to support a work platform above the water surface, as well as rigid and/or flexible piping extending from the work platform to the seafloor, where one or more drilling or well sites are located.
The hull typically comprises a pair of horizontal pontoons that support a plurality of vertically upstanding columns, which in turn support the work platform above the surface of the water. The size of the pontoons and the number of columns are governed by the size and weight of the work platform and its payload being supported.
A typical semi-submersible platform has a relatively low draft, typically, about 100 ft. (30.5 m), and incorporates a conventional catenary chain-link spread-mooring arrangement for station keeping over the well sites. The motions of these types of semi-submersible platforms are relatively large, and accordingly, they require the use of “catenary” risers (either flexible or rigid) extending from the seafloor to the work platform, and the heavy wellhead equipment is typically installed on the sea-floor, rather than on the work platform. The risers present a catenary shape to absorb the large heave and horizontal motions of the conventional semi-submersible platform. Due to their large motions, conventional semi-submersible platforms cannot support high-pressure, top-tensioned risers.
Typical semi-submersible offshore platforms are described in the following references: CA 1092601, GB 2,310,634, U.S. Pat. No. 4,498,412, WO 85/03050, GB 1,527,759, WO 84/01554, GB 2,328,408, U.S. Pat. No. 6,190,089, GB 1,527,759 and WO 02/00496.
It is known that increasing the draft of a semi-submersible platform can both improve its stability and reduce its range of movement. Doing so involves locating the pontoons at a greater depth below the surface of the water, where wave excitation forces are lower. Further, the area of the pontoons can be increased, resulting in the vessel having a greater hydrodynamic mass, and hence, resistance to movement through the water. Additionally, catenary mooring can be replaced by a so-called “taut leg” mooring system, further increasing the resistance of the platform to horizontal motion. Thus, a deep draft semi-submersible platform [i.e., having a draft of at least about 150 feet (about 45 m)] can have significantly smaller vertical and horizontal motions than a conventional semi-submersible platform, thereby enabling the deep draft platform to support top-tensioned drilling and production risers without the need for disconnecting the risers during severe storms.
In both conventional and deep draft types of semi-submersible platforms, the hull is divided into several closed compartments having a buoyancy that can be adjusted for purposes of flotation and trim, and includes a pumping system for pumping ballast water into and out of the compartments. The compartments are typically defined by horizontal and/or vertical bulkheads in the pontoons and columns. Normally, the compartments of the pontoon and the lower compartments of the columns are filled with water ballast when the platform is in its operational configuration, and the upper compartments of the columns provide buoyancy for the platform. The compartmentalization of the columns with bulkheads substantially increases the manufacturing costs of the platform, especially when a high degree of compartmentalization is effected.
Additionally, the methods by which the platforms are deployed for offshore operations are not optimal. In one known method, the hull (i.e., the pontoons and columns without the work platform mounted thereon) is transported to its operation site, either by towing it at a shallow draft, or by floating it aboard a “heavy lift” vessel. When the hull is at the operation site, it is ballasted down by pumping sea water into the pontoons and columns, and the work platform is then either lifted onto the tops of the columns by heavy lift cranes carried aboard a heavy lift barge, or by floating the work platform over the top of the partially submerged hull using a deck barge. In either case, the procedure is typically effected far offshore (e.g., 100 miles, or 161 km), is performed in open seas, and is strongly dependant on weather conditions and the availability of a heavy lift barge, making it both risky and expensive.
A second known deployment method involves installing the deck on the hull at the shipyard, then transporting the fully assembled semi-submersible platform to the operation site using a heavy lift vessel. This method is also strongly dependent on the availability of a heavy lift vessel. In yet another proposed method (see, e.g., international patent application WO 01/87700), a “stabilization module” is attached to the fully assembled platform to increase its water plane area and thereby stabilize it for towing to the operation site at a shallow draft. However, the use of a stabilization module increases the cost of the towing operation, and since the platform is towed with the relatively heavy deck mounted on top of the hull, this procedure also involves some risk.
Accordingly, there is a long-felt but as yet unsatisfied need for a semi-submersible offshore platform that incorporates adjustably buoyant support columns having a relatively high degree of compartmentalization, and yet which can be manufactured more simply and cost effectively. There is also a need for a method of deploying such a platform for offshore operations that is both less expensive and less risky than current platform deployment methods.